Production of high-purity niobium monoxide and capacitor production therefrom

ABSTRACT

The present invention relates to high-purity niobium monoxide powder (NbO) produced by a process of combining a mixture of higher niobium oxides and niobium metal powder or granules; heating and reacting the compacted mixture under controlled atmosphere to achieve temperature greater than about 1945° C., at which temperature the NbO is liquid; solidifying the liquid NbO to form a body of material; and fragmenting the body to form NbO particles suitable for application as capacitor anodes. The NbO product is unusually pure in composition and crystallography, and can be used for capacitors and for other electronic applications. The method of production of the NbO is robust, does not require high-purity feedstock, and can reclaim value from waste streams associated with the processing of NbO electronic components. The method of production also can be used to make high-purity NbO 2  and mixtures of niobium metal/niobium monoxide and niobium monoxide/niobium dioxide. The method further is ideal for doping of the product oxides to enhance particular characteristics of the materials. The method further allows the production of single crystal or directionally-solidified ingots. In contrast to the spongy, highly porous agglomerates produced by other techniques, the present invention produces solid, non-porous ingots that can be fragmented to fine, non-porous angular particles suitable for electronic applications.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/834,427, filed Apr. 29, 2004, which is a continuation-in-part of U.S.patent application Ser. No. 10/428,430, filed May 2, 2003, now U.S. Pat.No. 7,157,073.

FIELD OF THE INVENTION

The present invention relates to a method of producing niobium monoxidepowder of high purity, and the use of such niobium monoxide powders inthe production of valve devices, i.e., capacitors.

BACKGROUND OF THE INVENTION

It has been recognized that niobium monoxide (NbO) has some unusualelectrical properties that make it well-suited for the manufacture ofelectronic capacitors. It is of much lower flammability than equivalenttantalum powders, is less costly than tantalum, and has much largerpotential supply than tantalum. However, niobium monoxide capacitorpowders require high levels of purity, with not only foreign elementssuch as iron and copper being deleterious, but other forms of niobiumsuch as niobium metal, niobium dioxide (NbO₂), niobium trioxide (Nb₂O₃)and niobium pentoxide (Nb₂O₅) being harmful. In order to be useful in avalve application, the niobium monoxide must be in a finely dividedform, i.e., fine powder or, preferably, agglomerates formed from smallparticles, such small particles typically about 1-2 microns in diameteror finer. In order to meet these requirements, the electronics industryhas produced niobium monoxide by reacting agglomerated and sinteredniobium pentoxide or niobium dioxide (optionally pre-reduced from thepentoxide) with a metallic reducing agent under conditions in which theniobium oxides remain in the solid state. This allows the particlemorphology of the original agglomerated oxide to be preserved in theniobium monoxide. In one embodiment of this process, niobium pentoxideis reacted at temperatures of approximately 1000° C. with finely-dividedmetallic niobium, in such stoichiometric proportions as to produceprimarily niobium monoxide. In another embodiment, the niobium pentoxideor niobium dioxide is reacted with gaseous magnesium, again attemperatures of approximately 1000° C. This results in a spongy, highlyporous niobium monoxide-magnesium oxide mixture. After leaching themagnesium oxide, the resultant product is a porous, high-surface areaagglomerated mass of niobium monoxide.

Because of the low processing temperatures used in these methods ofproducing niobium monoxide, there is virtually no opportunity to removeany impurities in either the niobium oxide or the reducing agentfeedstocks. Moreover, impurities on the surface of the feedstockparticles remain on the surface through the solid-state processing,resulting in potentially detrimental concentrations of these impuritieson the surface of the NbO particles. The electronic characteristics ofcapacitors produced from such surface-contaminated particles may beseriously degraded. The purity requirements of the niobium monoxidedictate the purity required of the feedstock. The surface arearequirements of the product niobium monoxide dictate the particle sizedistribution and morphology of the niobium pent-or-di-oxide and niobiummetal needed for the process. These requirements severely limit theavailability of suitable raw materials. Further, because the reactionsoccur in the solid state, the reactions are sluggish and often do not goto completion. The product contains some higher oxides of niobium, andoften some niobium metal.

Thus, an object of the present invention is to produce niobium monoxide(NbO) powder of high purity and sufficient surface area to meet therequirements of NbO capacitors without the constraints of raw materialspurity and particle size imposed by solid-state processes, and the useof such powders in the production of capacitors. The present inventionalso can be used to produce high-purity niobium dioxide, and to producelarge, (non-particulate) non-porous objects of both niobium monoxide andniobium dioxide. The powders produced from such objects are non-porousand angular in shape.

SUMMARY OF THE INVENTION

The present invention relates to a high-purity niobium monoxide orniobium dioxide powder, produced by a process comprising:

(a) combining a mixture of niobium pentoxide, niobium trioxide, and/orniobium dioxide and coarse niobium metal powder in amountsstoichiometrically calculated to yield a product with a fixed atomicratio of niobium to oxygen, said ratio being close to about 1:1 in thecase of niobium monoxide, or about 1:2 in the case of niobium dioxide;

(b) forming a compact of said mixture by cold isostatic pressing orother techniques known to those skilled in the art;

(c) exposing said compact to a heat source sufficient to elevate thesurface temperature above the melting point of the product niobiummonoxide or niobium dioxide, i.e., greater than about 1945° C. forniobium monoxide or about 1915° C. for niobium dioxide in an atmospheresuitable to prevent uncontrolled oxidation;

(d) allowing the mixture to react exothermically to produce the desiredniobium monoxide;

(e) solidifying the liquid mixture to form a solid body of niobiummonoxide;

(f) fragmenting the body to form the desired particle size of niobiummonoxide; and,

(g) producing capacitor anodes from said niobium oxide particles bytechniques common to the capacitor industry.

For example, in order to produce niobium monoxide from niobiumpentoxide, the mixture of niobium pentoxide and metallic niobium wouldhave about a 1:1 ratio by weight. In order to produce niobium dioxidefrom niobium pentoxide, the mixture of niobium pentoxide and metallicniobium would have about a 5.7:1 ratio by weight.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a-c display the x-ray diffraction patterns for NbO produced bythe present invention (FIGS. 1 a-b) and NbO produced by a commercial,solid-state reaction (FIG. 1 c); and

FIG. 2 is an illustration of an ingot reduced to sharp, angular,substantially non-porous individual pieces.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a method of producing niobium monoxidepowder which includes combining a mixture of Nb₂O₅, Nb₂O₃, and/or NbO₂,and niobium metal; forming a compacted bar of the mixture; reacting themixture at a temperature greater than about 1945° C.; solidifying thereaction products; and fragmenting the solidified body to form niobiummonoxide powder. In a preferred embodiment of the present invention, theweight ratio of niobium pentoxide to niobium metal is about 1:1. Niobiumdioxide powder can be made in the same process by adjusting the ratio ofniobium pentoxide to niobium metal to about 5.7:1.

The present invention also relates to the production of a high-purityniobium monoxide or niobium dioxide powder produced by this process fromimpure niobium pentoxide and/or impure niobium dioxide, and from impureniobium metal powder. In the present invention, the high processingtemperature, controlled atmosphere and the presence of a liquid statecan be exploited to remove some major impurities, including iron,aluminum, and most other elements other than refractory metals.Impurities on the surface of the feedstocks (from crushing, grinding,milling, etc.) are dissolved into the liquid NbO, producing a uniformdistribution throughout the particle and thereby reducing the harmfuleffects of such impurities. The liquid state processing also allowsother, desirable elements to be added to the product.

The solid ingot produced by the present invention can be sized to anydesired size by comminution techniques well known to those skilled inthe art. This allows production of sizes from the ingot down tosub-micron particles. Moreover, coarse particles of niobium monoxide orniobium dioxide can be used as milling media to produce fine powdersfree of the contamination introduced by ordinary milling media.

Example 1

In the testing of the present invention, a mixture ofcommercially-available 99.99% pure Nb₂O₅ and commercially-availableelectron-beam triple-refined dehydrided niobium metal powder (50×80 USmesh) was blended and formed into a bar by cold isostatic pressing,although other means of compaction and resultant physical forms areacceptable. Three such bars were prepared.

The compacts of Nb₂O₅ and niobium metal (weight ratio 1:1.05) were eachfed sequentially into the melting region of an electron beam vacuumfurnace, where each compact reacted and liquefied when heated by theelectron beam, with the liquid product dripping into a cylindricalwater-cooled copper mold. When the electron beam initially struck thecompact melting immediately took place, with only a small increase inchamber pressure. With experience, the production rate easily reached100 pounds an hour. Reaction was terminated before the final compact hadbeen fully consumed, leaving a layer of partially-reacted materials onthe face of the residual compact.

While an electron-beam furnace was used in this experiment, it isobvious to those skilled in the metallurgical arts that other energysources capable of heating the materials to at least 1945° C. could alsobe used, including, but not limited to, cold crucible vacuum inductionmelting, plasma inert gas melting, vacuum arc remelting, and electricalimpulse resistance heating.

The method of the present invention provides an opportunity to add awide range of dopant materials to the mixtures prior to compaction, suchadditions melting into the liquid melt during the melt-reaction process.Such dopants include, but are not limited to tantalum, titanium,vanadium, and aluminum. Such dopants may be added in amounts up to 40%by weight. While the usual purpose of dopants is to improve the specificcapacitance of capacitor materials, they may provide other advantages,such as improved long-term stability and reduced DC leakage.

A further advantage of the present invention relates to the form of theingot so produced. By applying well-known metallurgical principles, itis possible to produce a single-crystal or directionally-solidifiedingot that may offer advantages in applications beyond conventionalcapacitor powders.

The resultant ingot was allowed to cool under vacuum, and the apparatuswas vented to atmosphere. The ingot was a solid, non-porous cylinder.The ingot was subsequently shattered by impact. Samples were taken fromthe top one inch of the ingot (the “top” samples), while “edge” sampleswere taken from lower mid-radius locations in the ingot.

Subsequent analysis of the product NbO samples by x-ray diffractionshowed a clean pattern for NbO, with no additional lines attributable toniobium metal, NbO₂ or Nb₂O₃. In FIG. 1, the x-ray diffraction patternsare shown for NbO produced by the present invention (FIGS. 1 a-b), andNbO produced by a commercial solid-state reaction (FIG. 1 c). Thesolid-state reaction product has numerous lines not originating withNbO, indicating the presence of other, undesirable phases. Gravimetricanalysis showed the material to be stoichiometric NbO, within the limitsof analytical precision.

It will be apparent to those skilled in the art that alterations in theinitial powder mixture allow the production not only of high-purityniobium monoxide, but also of high-purity niobium dioxide, and furtherof intimate mixtures of niobium metal/niobium monoxide or niobiummonoxide/niobium dioxide, as illustrated in the Niobium—Oxygen phasediagram (see, “Binary Alloy Phase Diagrams”, American Society forMetals, Metals Park, Ohio, 1990, p. 2749).

The ingot was then taken down to powder by conventional crushing,grinding and milling techniques. Upon crushing the ingot was reduced tosharp, angular, non-porous individual pieces, as illustrated in FIG. 2.The morphology of these pieces was retained by individual particles downto sub-micron sizes. The resultant NbO powder had a Microtrac D50 of2.38 microns and a B.E.T. surface area of 2.06 m²/gram. When formed intoa capacitor anode under conventional conditions, (Forming Voltage 35 V;Forming current 150 mA/g, sintered at 1400° C.) the anodes showedspecific capacitance at a 2-volt bias of 60,337 CV/g and a DC Leakage of0.31 nA/CV. Tested with a 0 volt bias, the specific capacitance was78,258 CV/g and the DC Leakage was 0.23 nA/CV. These values are wellwithin the normal range for commercial capacitors produced from NbO madeby solid-state reactions, as well as some tantalum capacitors.

Example 2

Four additional experimental runs were performed using less purefeedstock and altering the sizing of the feedstock used to make thecompacts. In each case, the product was NbO free of other compounds andfree of metallic niobium. This indicates the subject process is robustand not dependent on particular sources of oxides or niobium metal. Inone experimental run, the commercial-grade niobium pentoxide used asfeedstock contained approximately 400 ppm of iron, and the niobium metalcontained less than 50 ppm of iron. After converting the feedstock toNbO by the subject process, the NbO was analyzed and found to containless than 100 ppm of iron. This represents a reduction of at least 50%in the iron content during the subject process. The subject process alsooffers the opportunity to recover NbO values from waste streamsassociated with production of powder-based NbO products, since therefining action of the present invention can effectively remove ordilute most contaminants, even when such contaminants are present asfine or micro-fine powders or particles.

The NbO ingot from each of these four additional experimental runs wasreduced in size by conventional crushing, grinding and milling to anaverage particle size under 2.5 microns, formed into test anodes, andtested for capacitance and leakage rates. The results in each case weresimilar to the initial results described above, including anodesproduced from NbO originating from the high-iron feedstock as notedabove. The specific capacitance and DC leakage of NbO powder producedfrom such ingots were 69,200 CV/g and 0.34 nA/CV, respectively. Whilethe iron level would normally be considered too high to permit good DCleakage values, in these examples the iron has been uniformlyre-distributed throughout the particles. This re-distribution results ina very low level of iron on the particle surfaces, so that the iron doesnot degrade the leakage characteristics of the NbO.

Example 3

The formation of niobium monoxide by melt phase processing lends itselfto the recovery and remelting of niobium monoxide solids in, but notlimited to powder, chips, solids, swarf and sludges. Off-grade powder,recycled capacitors and powder production waste are among the materialsthat can be reverted to full value niobium monoxide by this process. Acompact was prepared from “waste” NbO powders of various sizes andproduction states. The compact was melt-reacted in the electron beamfurnace to produce a sound NbO ingot. Subsequent testing of the ingotshowed it to be indistinguishable in crystalline structure, purity, andelectronic characteristics (specific capacitance, DC leakage) fromearlier ingots produced from high-purity raw materials. Glow DischargeMass Spectrometry showed no elevated impurity levels compared to earlier“high-purity” ingots.

Example 4

Niobium pentoxide and metallic niobium powder were mixed in proportionscalculated to produce niobium dioxide, and the mixture compacted andmelt-reacted in the electron beam furnace as described above. The ingotwas sound, solid, and showed no obvious defects. A sample taken from theingot was analyzed to determine the ratio of oxygen and niobium. Withinthe limits of analytical precision, it was stoichiometric NbO₂. NbO₂theoretically contains 25.13% oxygen by weight. The NbO₂ of this exampleanalyzed 25.14% oxygen.

Although the invention has been described and illustrated with referenceto specific illustrative embodiments thereof, it is not intended thatthe invention be limited to those illustrative embodiments. Thoseskilled in the art will recognize that variations and modifications canbe made without departing from the spirit of the invention. It istherefore intended to include within the invention all such variationsand modifications which fall within the scope of the appended claims andequivalents thereof.

1. A method of producing niobium monoxide ingots or powder whichcomprises: a) combining a mixture of (1) a niobium oxide selected fromthe group consisting of Nb₂O₅, NbO₂, and/Nb₂O₃, and (2) metallicniobium, wherein the niobium oxide and metallic niobium are present inpowder or granular form; b) forming a compact of said mixture; c)reacting the mixture with a heat source such that a temperature greaterthan 1945° C. is achieved; d) solidifying the reacted mixture to form abody of material; and e) fragmenting the body of material to form theNbO powder.
 2. The method as recited in claim 1, wherein the mass ratioof Nb₂O₅ to niobium metal powder or granules in the mixture is about1:1.
 3. The method as recited in claim 1, wherein the mass ratio of NbO₂to niobium metal powder or granules in the mixture is about 1.3:1. 4.The method as recited in claim 1, wherein the mass ratio of Nb₂O₃ toniobium metal or granules in the mixture is about 2.5:1.
 5. The methodas recited in claim 1, wherein the niobium oxide is Nb₂O₅.
 6. The methodas recited in claim 1, wherein the reaction achieves a temperature of1945° C. or greater.
 7. The method as recited in claim 1, wherein theheat source is an electron beam furnace.
 8. The method as recited inclaim 1, wherein the heat source is a plasma-arc furnace.
 9. The methodas recited in claim 1, wherein the heat source is an induction furnace.10. The method as recited in claim 1, wherein the heat source is avacuum arc remelting furnace.
 11. The method as recited in claim 1,wherein the mixture further comprises niobium monoxide revert, niobiummetal lead wire, or other niobium-containing waste products.